The evolution of man : a popular scientific study / by Ernst Haeckel. Translated from the 5th (enl.) ed. by Joseph McCabe.

  • Haeckel, Ernst, 1834-1919.
Date:
1905
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Credit: The evolution of man : a popular scientific study / by Ernst Haeckel. Translated from the 5th (enl.) ed. by Joseph McCabe. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

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    conjecture than knowledge. Its intri- cate structure corresponds to the very complicated functions of the mind. Never- theless, this elementary organ of psychic activity—of which there are thousands in our brain—is nothing but a single cell. Our whole mental life is only the joint result of the combined activity of all these nerve-cells, or soul-cells. In the centre of each cell there is a large transparent nucleus, containing a small and dark nuclear body. Here, as elsewhere, it is the nucleus that determines the indivi- duality of the cell ; it proves that the whole structure, in spite of its intricate composition, amounts to only a single cell. In contrast with this very elaborate and very strictly differentiated psychic cell (Fig. 9), we have our ovum (Figs, i and 2), which has hardly any structure at all. Fig. 8.—Unfertilised ovum of an echinoderni (from Hertivig). The vesicular nucleus (or germinal vesicle) is globular, half the size of the round ovum, and encloses a nuclear framework, in the central knot of which there is a dark nucleolus (the germmal' spot). But even in the case of the ovum we must infer from its properties that its proto- plasmic body has a very complicated chemical composition and a fine molecular structure which escapes our observation. This presumed molecular structure of the plasm is now generally admitted ; but_ it has never been seen, and, indeed, lies far beyond the range of microscopic vision. It must not be confused—as is often done—with tiie structure of the plasm (the fibrous net-work, groups of granules, honey-comb, etc.) which does come within the range of the microscope. But when we speak of the cells as the elementary organisms, or structural units, or ultimate individualities, we must bear in mind a certain restriction of the phrases. 1 mean, that the cells are not, as IS often supposed, the very lowest stage of organic individuality. There are yet more elementary organisms to which I must refer occasionally. These are what we call the cytodes (y/'oi'= cell), certain liwng, independent beings, con- sisting only of a particle of plasson—an albuminoid substance, which is not yet differentiated into carj'oplasm and cyto- plasm, but combines the properties of both. Those remarkable beings called the monera — especially the chromacea and bacteria — are specimens of these simple cytodes. (Compare the nineteenth Chapter.) To be quite accurate, then, we must say : the elementary organism, or the ultimate individual, is found in two different stages. The first and lower stage is the cytode, which consists merely of a particle of plasson, or quite simple plasm. The second and higher stage is the cell, which is already divided or differentiated into nuclear matter and cellular matter. We comprise both kinds —the cytodes and the cells—under the name of plastids (formative particles ), because they are the real builders of the organism. However, these cytodes are not found, as a rule, in the higher animals and plants ; here we have only real cells with a nucleus. Hence, in these tissue- forming organisms (both plant and animal) the organic unit always consists of two chemically and anatomically dif- ferent parts—the outer cell-body and the inner nucleus.' In order to convince oneself that this cell is really an independent organism, we have only to observe the development and vital phenomena of one of them. We see then that it performs all the essential functions of life—both vegetal and animal —which we find in the entire organism. Each of these tiny beings grows and nourishes itself independently. It takes its food from the surrounding fluid some- times, even, the naked cells take in solid particles at certain points of their surface —in other words, eat them—without needing any special mouth and stomach for thepurpo.se (cf. Fig. 19). Furtiier, each cell is able to reproduce itself. This multiplication, in mo.st cases, takes the form of a simple cleavage, sometimes direct, sometimes indirect; the simple direct (or amitotic) division is less common, and is found, for instance, in the blood cells (Fig. 10). In these the nucleus first divides into two equal parts byconstriction. The indirect (or mitotic )